Implement Montana State Supplementary Payment (SSP)

## Goal

Implement Montana's State Supplementary Payment (SSP) — Montana's supplement to federal Supplemental Security Income (SSI).

## Scope

- Add Montana SSP parameters under `parameters/gov/states/mt/`
- Add Montana SSP variables under `variables/gov/states/mt/`
- Add YAML tests under `tests/policy/baseline/gov/states/mt/`
- Register the program in `programs.yaml`
- Wire the SSP payment into household state benefits (`spm_unit_benefits.py`, `household_state_benefits.yaml`)

## Regulatory Authority

To be confirmed during implementation (Montana DPHHS / state statutes / SSA SSI State Factsheets).

## Notes

- Eligibility typically requires receipt of federal SSI (`ssi > 0`) plus state residency.
- Verify benefit amounts and any categorical distinctions (aged/blind/disabled, living arrangement, couple treatment) against primary sources.